While your bike may be worth $800, I highly doubt you'd have luck finding someone willing to fork over that much for a 3yo mtb. As said before, it depends on market conditions. My guess is you'd be lucky to get $400 for it. However, if your components are XT/XTR and you have a good wheelset right now, you might be able to get more.
You could buy a set of slick MTB Tires for $50+/- and mount them on your existing wheels. Or you could buy a whole new wheelset to make changing them over easier. Wheesets jump in price quick, and make sure you factor in getting them true'd and tensioned.
I happen to have two wheelsets and would say that a MTB with narrower / slick tires is not equal to a Roadbike. It's just a MTB that has less resistance on the road. If you plan to race and/or ride with Roadies, then you are going to want a roadbike.
TIP: There are some bike shops that sell on consignment, that might help you get a little more and save some hassles... bring it in and give them your bottom line price and see if they'll sell it for you. If you come back a month later and it's still for sale, the price is too high.
